Thermal Modeling and Evaluation of Large Transformer Tank

Abstract

Transformer thermal performance impacts transformer life and reliability through degradation of insulation and increased losses; hence it is a critical issue in design aspects. In previous decades maximum ratings of the large transformers were 315 MVA / 400 kV, which has increased to 500 MVA / 800 kV. As power system capacity is growing, higher rating transformers are being purchased by the utilities. On the other hand manufacturers are also kept under market pressure to optimize the design leading to large risks of failure if not properly analyzed. Thermal behavior of transformer depends on magnetic shunt provided to inner surface of tank wall, leakage flux density, viscosity of oil (which changes with temperature), physical parameters of applied materials, geometry of core and windings, type of cooling and heat transfer. Objective of this project is to reduce Hot-Spot Temperature (HST) on Tank Wall (side walls of the main tank)-Flange Bolt region (bolted joint of tank and cover) and Yoke Clamp (support for yoke beams). Due to lack of magnetic shielding and non-linearity characteristics of magnetic field, stray loss occurs in flange bolt region, which is needed to be minimized to maintain the reliability of transformer. Meanwhile the cooling of transformer will be considered to reduce the temperature of tank wall. Calculation of stray losses is complex due to non-linearity of magnetic field, inability of isolating exact stray loss components from tested load loss values. Such calculation can also be done by Finite Element Method. FEM enhances the ability to represent transformer characteristics and performance, has been used in the work to measure temperature distribution on tank walls. Implementations of 2-D/3-D FEM analysis to a typical large generator transformer has been carried out in Ansoft - MAXWELL for the project to predict (a) eddy and stray current losses at flange bolt region and yoke clamp joints. Analysis results of Ansoft Maxwell software are imported in ANSYS Workbench software to visualize the temperature distribution at the flange bolt region and yoke-clamp joints. Results of ANSYS Workbench results have been verified by the actual values of test results of the same transformer tested in the UHV Test Lab at LTI, Alstom Grid India Ltd, Vadodara. By this work, conclusion has been derived that magnetic shielding can be provided at the flange bolt region of the transformer to bypass the magnetic field. Another conclusion for yoke-clamp has been derived that during manufacturing process of large power transformer, tightening of the yoke-clamp bolt must be check and confirmed with standards otherwise it may lead to hot-spot formation at the joints of yoke-clamps. In this thesis, physics of large transformer and its thermal model are considered for evaluation of large transformers.